Thickness and hardness measurement apparatus utilizing a rotating induction vector

ABSTRACT

Apparatus for measuring the sidewall thickness of cylindrical articles during in-process ram and die drawing, inclined axis multiple polar sensors induce eddy currents by means of the angular motion of a rotating magnetic field, the signal phase angle representing sidewall thickness, also ram and die centering signals generated. Inclined axis polar sensors also measure both cylindrical and flat article hardness by phase shift.

This application is a continuation in part of copending patentapplication Ser. No. 08/142,933 entitled "Apparatus for detectingsurface flaws in cylindrical articles by means of asymmetric magneticdetection" filed Oct. 29, 1993, by the present applicant. The conceptionof the present invention was recorded in Disclosure Document No. 369233filed Jan. 30, 1995.

1. Field of the Invention

The invention relates in general to ferrous or non-ferrous materialthickness measurement, and more specifically to a thickness sensorutilizing a rotating magnetic field.

2. Description of the Prior Art

Conventional thickness measurement apparatus have utilized analternating field (collapsing field) principle to induce eddy currentsin the metal workpiece. In measurment applications, variations in thespacing between the probe and the material produce an undesirable signalcomponent which significantly affects the accuracy and reliability ofthe eddy current test results. The undesired signal is known as probespacing, probe wobble, or lift-off. Hereinafter, the undesired signal isreferred to as "lift-off". Sensor to workpiece distance variationgenerates a corresponding thickness error. Much effort has been made toreduce this measurement error, i.e. phase rotation circuitry such astaught in the Denton et al. U.S. Pat. No. 4,424,486. The presentinvention eliminates the need for additional phase rotation circuitrysince thickness and hardness is phase shift transduced directly by thepolar sensor.

SUMMARY OF THE INVENTION

A very important factor in the manufacture of two-piece aluminum cans issidewall thickness. Recent efforts have been made to monitor sidewallthickness during the can drawing process by means of collapsing fieldeddy current sensors. An object of the present invention is to providean in-line aluminum can sidewall thickness measurement apparatusgenerating a phase shift signal corresponding to the sidewall thickness.Firstly, the present invention is a thickness measurement apparatusutilizing a rotating magnetic field within a hollow toroid core. Thefirst embodiment of a cylindrical article sidewall thickness measurementapparatus utilizes the multiple sensor driving capability of the hollowtoroid driving core to scan the cylindrical article at four longitudinallocations i.e. x-y axes. This x-y scanning may also be utilized for ramand die centering control. This apparatus measures the thickness of bothferrous and non-ferrous cylindrical articles. The instant inventionutilizes the polar coordinate sensor (polar sensor) disclosed in Ser.No. 07/842,244 filed Feb. 27, 1992, now U.S. Pat. No. 5,404,101,entitled "Rotary sensing device".

I also utilized polar coordinate sensors in my patent application Ser.No. 08/142,933 filed Oct. 29, 1993, entitled "Apparatus for detectingsurface flaws in cylindrical articles by means of asymmetric magneticdetection". The above cited invention utilized the polar coordinatesensor sensing pattern as a 360 degree balanced flux detector. The polarcoordinate sensor indicates target direction by the phase angle of thesignal, and target distance by the amplitude of the same signal. In thepresent invention the axis of the polar sensor is inclined at an angleto the surface of the workpiece, this inclined rotating sensing patterninduces eddy currents in the container sidewall. This unbalanced fluxpattern links the pick-up coil generating a sine wave signal in whichthe phase angle varies with the workpiece thickness and amplitude varieswith sensor to workpiece spacing. Eddy current induction is by theangular motion of a constant length flux vector. The depth of the eddycurrent i.e. thickness of the workpiece is the major phase variable whenused in this mode. The azimuth heading of the workpiece is the sameregardless of sensor to workpiece spacing and therefore the minor phasevariable. This means "lift-off" errors are greatly reduced. Sensor toworkpiece spacing is the major amplitude variable of polar coordinatesensing, material thickness or hardness being the minor amplitudevariable. These amplitude variations may be utilized in a host computerto compensate for minor phase errors due to sensor to workpiece distancevariations. This error compensation is performed in the signalprocessing after amplitude peak detection and zero crossing detection.The inclined axis polar sensor also measures the hardness of ferrousmetals by phase shift. A phase shift signal being time dependent, ispreferred over an amplitude modulated signal. The principle advantage ofmeasuring thickness or hardness by phase difference is that theamplitude of the signal involved is irrelevant and the circuitry neededto convert a phase difference to a digital signal is morestraightforward than that needed to compare signals with differentamplitudes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood, and further advantages and usesthereof more readly apparent, when considered in view of the followingdetailed description of exemplary embodiments taken with theaccompanying drawings in which:

FIG. 1 is a perspective view of a hollow toroid core, having a mountedpolar coordinate sensor (PS) which is the fundamental sensing elementutilized in the disclosed thickness and hardness measurement apparatus.

FIG. 2 is an isometric view of a polar sensor (PS).

FIG. 3 is a radial view of a hollow toroid core which illustrates theflux lines coupling an inclined target sheet.

FIG. 4 is a radial view of a multiple polar sensor assembly (MPSA) withtwo polar sensors on the X-axis and two on the Y-axis.

FIG. 5 is a longitudinal view of an in line aluminum can drawing processwith the multiple polar sensor assembly concentrically surrounding thealuminum can for sidewall thickness measurement.

FIG. 6 is a radial view of the annular shaped polar sensing faceillustrating eddy current induction in the can sidewall by the angularmotion of a constant length rotating induction vector.

FIG. 6A is a graph illustrating signal phase deviation corresponding tosidewall thickness.

FIG. 7 is a 360 degree phase diagram illustrating ferrous andnon-ferrous azimuth locations relating to the polar sensing face.

FIG. 7A is the sine-cosine reference signal.

FIG. 7B illustrates non-ferrous thickness phase-shift response.

FIG. 7C illustrates ferrous thickness phase-shift response,

FIG. 7D illustrates ferrous metal hardness phase-shift response.

FIG. 8 is a cross sectional view of a conical sensing face polar sensor.

FIG. 8A is a cross-section view of a poly-phase polar sensor having aconical sensing face.

FIG. 9 is a perspective view of a conical sensing face polar sensor.

FIG. 9A is a perspective view of a poly-phase polar sensor having aconical sensing face.

FIG. 10 is a radial view of a container being scanned on x-ylongitudinal axes.

FIG. 11 is a longitudinal view of a in-process clylindrical articlesidewall thickness measurement apparatus according to the invention.

FIG. 12 is the sine-cosine waveforms used to excite the driving core.

FIG. 13 is a schematic of the inside and outside excitation windings.

FIG. 14 is a schematic if the poly-phase polar sensor pick-up coils.

FIG. 15 is an example signal processing circuit for use with thepreferred embodiments.

DEFINITIONS AND ABBREVIATIONS

Driving core: a hollow toroid core generating a rotating magnetic field,used to drive one or more polar sensors.

Polar sensor: (PS) a rotating field sensor generating a sine wave signalhaving polar coordinate components i.e. the signal phase angle indicates(1) target direction, (2) target material i.e. ferrous or non-ferrous,(3) target thickness, (4) target hardness. The amplitude of this samesignal indicates target distance, (2) target thickness.

Multiple polar sensor assembly: (MPSA) a hollow toroid driving corehaving more than one mounted polar sensor, for generating a plurality ofpolar coordinate signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and to FIG. 1 in particular, there isshown in perspective view a hollow toroid core 55 formed of aferromagnetic material such as ferrite. The hollow toroid 55 has aninside excitation winding 58, having connecting leads 59. Winding 58 caninduce a magnetic field throughout the entire core. There is an outsidetoroidal excitation winding 62 having connecting leads 63, this windingcan also induce a magnetic field throughout the entire core. The insideand outside excitation windings are connected to two pure sine wavevoltage sources (not shown) displaced by 90 degrees. In the preferredsine-cosine generator the two waveforms are constructed by digitalmeans. If the sine wave voltage sources are exactly 90 degrees out ofphase a rotating magnetic field is induced throughout the hollow toroidcore. This rotating magnetic field has distributive axes perpendicularto the surface of the core 55 at all points. This uniform rotating fieldmay be utilized for eddy current sensing by mounting a polar coordinatessensing element PS in a bore 92 disposed in the outer circumference wallof the hollow toroid. The axis of this bore being perpendicular to thecentral axis of the hollow toroid 55, e.g. perpendicular to a planedrawn tangent to the surface of the hollow toroid 55. In this positionthe pick-up coil of FIG. 2 (polar sensor shown in enlarged detail) ispositioned coplaner to the rotating flux lines crossing the mountedpolar sensor PS. When no target is present there is no flux coupling tothe coplaner positioned pick-up coil, and a signal null is obtained.When a metallic target 83 comes within the hemispherical sensing patternas in FIG. 3, this coplaner flux is unbalanced linking flux lines topick-up coil 90 generating a sine wave signal. FIG. 2 is an isometricview of polar sensor PS showing the concentric construction. Pick-upcore 88 comprises, a central magnetic pole 86 concentrically surroundedby a cylindrical outer magnetic pole 84. The central and outer magneticpoles are concentrically spaced apart to provide an annular pick-up coilspace 89. The mentioned pick-up coil 90 is wound around the centralmagnetic pole 86. The magnetic poles and pick-up coil share the sameZ-axis. Pick-up core 88 is formed of a ferromagnetic material such asferrite; a conventional pot core half without lead slots was used in myprototype. The central and outer magnetic poles are connected by a baseportion 85. The thickness of the base portion 85 is approximately onehalf the thickness of the hollow toroid wall. When mounted in bore 92pick-up core 88 creates an annular shaped high reluctance area in thehollow toroid wall. This annular shaped high reluctance provides ahemispherical fringing flux sensing pattern. This hemispherical sensingpattern is rotating coaxially with the Z-axis of the pick-up core 88.

The pick-up core 88 is tightly mounted in bore 92 for good flux couplinge.g. a good machined fit. In FIG. 3 a small portion of pick-up core 88extends beyond the surface of the hollow toroid, this providing a moredefined sensing pattern to the workpiece. The unsegmented outercylindrical magnetic pole 84, of pick-up core 88, provides a veryuniform hemispherical sensing pattern that allows a very high angularresolution i.e. phase shift resolution. The pick-up coil connectingleads 91 may be routed out through a small hole in the base portion 85and from there out through a hole in the wall of the hollow toroid.

Several factors determine the exact phase angle of the generated signali.e. (1) target azimuth direction, (2) target material (ferrous ornonferrous), (3) conductivity, (4) target thickness, and (5) targethardness. The last two factors (thickness and hardness) were notdetected by the inventor at the dates of my previous patentapplications; state of the art sine-cosine generators and signalprocessing equipment has revealed these moderate phase deviations.Referring again to FIG. 3, the rotating induction vector within thehollow toroid driving core 55 causes flux lines 80 to flow up throughthe outer concentric magnetic pole 84, into the metal plate target 83,down through the central magnetic pole 86, through the base portion 85,and back to the driving core 55. Notice the metal plate target 83 isinclined at a slight angle to the polar sensing face, this causes therotating sensing pattern to become unbalanced generating the outputsignal. This inclined axis mode is utilized in the disclosed apparatusesto measure article (both cylindrical and flat sheet) thickness andhardness.

The first embodiment of an in-process can sidewall thickness measurementapparatus 100 is shown in FIGS. 4, 5. During manufacture, thin sheets ofaluminum are pushed through a series of dies 110 by a cylindrical ram111. Positioned just beyond the final die in FIG. 5 is a multiple polarsensor assembly (MPSA) comprising, a hollow toroid core 55 formed of ahigh permeability ferromagnetic material such as ferrite.

There is an inside excitation winding 58 (dashed lines), with connectingleads 59. There is also an outside toroidal excitation winding 62 havingconnecting leads 63. The outside excitation winding 62 may be subdividedas shown into four sub-coils (parallel or series connected) forimpedance matching. The sub-coils must be wound symmetrical to themounted polar sensors for flux balance as shown in FIG. 4. Four polarsensors (PS) are mounted in four bores in the lesser circumference wallon X-Y axes. Although four longitudinal sensing positions are deemedenough, twice the shown number of mounted polar sensors may be used e.g. a polar sensor every 45 degrees around the can sidewall. One drivingcore can drive several mounted polar sensors simultaneously. Thicknessmeasurement on X-Y axes provides control of ram and die centering. Thesensing faces of the polar sensors PS are toward the aluminum can 112,in this way the rotating flux pattern couples to the can sidewall at aninclined angle inducing eddy currents. The sensing flux path is shown inFIG. 5, where the rotating flux flows from the hollow toroid drivingcore 55 through the outer cylindrical magnetic pole 84, into the cansidewall 112, through the central magnetic pole 86, through the baseportion 85, and back to the hollow toroid core. Notice in FIG. 5, thehollow toroid 55 has an elliptical cross section to provide a near flatportion to mount the polar sensors. The major elliptic axis 106, and theminor elliptic axis 107 are inclined to the longitudinal axis AA ofcontainer 112. Also the Z-axis of the mounted polar sensors are inclinedat an acute angle 93 (3-15 degrees) to a plane 95 perpendicular to thelongitudinal axis AA of the container 112. This slight inclination ofthe polar sensor Z-axis relative to the can sidewall, unbalances thehemispherical sensing pattern, providing flux linkage to the pick-upcoils, and generates a sine wave signal in each of the four pick-upcoils 90. The central axis BB of the hollow toroid core 55 is disposedcoaxially with the longitudinal axis AA of the aluminum can 112.Although an ellipital hollow toroid core cross-section has been shown, aparallelogram shaped cross-section is topologically equivalent to ahollow toroid core, in which the axis of the bores are inclined at anangle to a plane drawn perpendicular to the central axis of the hollowtoroid core. It is contemplated this multiple polar sensor assembly(MPSA) may be rigidly mounted in the can stripper finger mechanism (notshown) by means of an encapsulating compound such as Epoxy. Tests haveshown that a ferrous plate on the far side (test aluminum sheet betweenthe ferrous plate and the sensing face) of the test aluminum sheet doesnot affect the thickness measurement. In view of this, a carbide ram 111inside the aluminum can should not hinder the sidewall thicknessmeasurement. The hollow toroid core 55 is made separable into two halvesfor assembly. It is contemplated the core separation may be on eitherthe major or minor ellipital axes. Referring now to FIG. 6, to explainthe thickness measurement principle utilized in the invention. FIG. 6 isan isometric radial view of the polar sensing face, showing thecylindrical outer magnetic pole 84, the central magnetic pole 86, andthe pick-up coil 90. The Z-axis of polar sensor PS is shown angled tothe cylindrical aluminum article 87AL (a fragmentary view of a containerwall) for unbalanced flux coupling. The polar sensing face is laid outin four quadrants for azimuth reference. The constant length rotatingflux vector coupling across the cylindrical outer pole of the pick-upcore 88 acts as a annular shaped permanent magnet 96 spinning at ultrahigh speed. Eddy current induction is by angular field motion incontrast to the collapsing field of conventional eddy current thicknessgauges. The 270 degree azimuth of PS is proximate to sidewall ofcylindrical non-ferrous article 87AL, thus the induced eddy currentsrepel the sensing pattern toward the 90 degree azimuth, but theresultant phase angle depends also on the sidewall thickness ofcontainer 87AL. FIG. 6A illustrates how the signal phase angle shiftswith changing target thickness. To explain the reasoning behind"lift-off" error reduction, refer again to FIG. 6, the 270 degreeazimuth heading remains the same regardless of variations in sensor toworkpiece spacing 98. Tests have shown "lift-off" error is reduced by asubstantial factor. Tests have also shown the phase deviation in the0.004"-0.006" thickness range has at least 3 degrees phase shift per0.001" of article thickness. If the individual sine-cosine excitationsignals are of equal magnitude and out of phase exactly 90 degrees, thetip of the induction vector traces out a circle. This circular inductionvector is coupled to the workpiece by the unsegmented pick-up core 88providing a sensor having near infinite angular resolution. This nearinfinite angular resolution provides a very high thickness resolutionand accuracy. The hollow toroid driving core 55 may be operated atsaturation level, eliminating hysteresis losses, (refer to theDelVecchio et al. U.S. Pat. No. 4,595,843 col. 3 line 39). Alsooperating at saturation level increases the sensing range. Thesine-cosine excitation waveforms are shown in FIG. 12. FIG. 13 is aschematic of the inside 58 and outside 62 excitation windings. Referingnow to FIG. 7, which is a 360 degree phase diagram illustrating thephase shift response of the invention to both ferrous and non-ferrousmetal sheets. This circular diagram relates to polar sensing faceazimuth position (reference degrees are arbitrarily chosen forconvenience). The shaded area 102 is the phase deviation range fornon-ferrous sheets having progressively thicker dimensions. As thenon-ferrous sheets get thicker the phase angle lags as shown in FIG. 7B.The sine-cosine excitation reference signal is shown in FIG. 7A. Theshaded area 101 is the phase deviation range for ferrous sheets havingprogressively thicker dimensions. As the ferrous sheets get thicker thephase angle leads as shown in FIG. 7C. The phase deviation ranges shownin FIG. 7 are only approximate since eddy current penetration depthdepends on field rotation speed i. e. sine-cosine excitation frequency.It is contemplated selectable frequency excitation sources be used withthe disclosed embodiments.

Although the phase deviations shown are modest, they can be easilydetected with well known zero crossing detector circuits and processedto a very high resolution. This very high thickness resolution may beutilized to control the sidewall thickness to a high degree of accuracy.Ram centering (x-y axes) may also be monitored and controlled bysuitable processing of the polar signals. A second embodiment of anin-process can sidewall thickness measurement apparatus 200 is shown inFIGS. 10, 11 (the forming dies and ram are not shown in these drawings).As in the first embodiment the polar sensors are postioned just beyondthe final die. Sidewall thickness measurement apparatus 200 utilizes thesingle polar sensor embodiment of FIGS. 1, 2, 3. The polar coordinatesensor principles will not be repeated here for brevity (same referencenumbers). As seen in FIG. 10, four polar sensors PS are arranged on X-Yaxes to scan the container sidewall 112 as it emerges from the finaldie. Although four longitudinal sensing positions are deemed enough,twice the shown number of polar sensors may be used i.e. a polar sensorevery 45 degrees around container 112. It is contemplated the polarsensors PS may be rigidly mounted in the aforementioned stripper fingermechanism 103 by means of an encapsulating compound 104. As in the firstembodiment, the Z-axis of the mounted polar sensors are inclined at anangle 93, (3-15 degrees) to the longitudinal axis AA of container 112for unbalanced flux coupling to the can sidewall to generate eddycurrents therein. The sensing flux path is the same as the firstembodiment of FIG. 5. For improved inclined axis flux coupling to theworkpiece, a conical sensing face polar sensor is shown in FIGS. 8, 9.This embodiment is the same as the embodiment of FIG. 2, except theouter cylindrical 84A and central 86A magnetic poles have a conicalshape. The conical angle CC should correspond to the axis inclinationangle of the mounted polar sensors. The rotating induction vector 81, isshown in FIG. 9. FIGS. 8A, 9A, show a third embodiment of polar seningelement e.g. a poly-phase conical sensing face polar sensor having twopick-up coils 90 and 90A. This poly-phase version uses principlesdisclosed in my parent patent application Ser. No. 07/842,244. Thepoly-phase pick-up core 88B has two pick-up coil spaces 89, and 89A withcorresponding pick-up coils 90, and 90A, having connecting leads 91, 91Arouted out through two small holes in the base portion 85. The conicalsensing face is for the same purpose as the single-phase polar sensor ofFIGS. 8, 9, i.e. to improve the inclined axis flux coupling to target83. The two signals generated by pick-up coils 90 and 90A, may be usedto extract more thickness or hardness information. Poly-phase pick-upcore 88B couples flux in two slightly different axial positions 108, 109to the can sidewall. If the can sidewall is not a perfect cylinder i.e.axially out of alignment, there will be a small phase difference betweenthe signals generated by pick-up coils 90 and 90A. Although the twoembodiments of in-process thickness measurement apparatuses have beenshown inspecting aluminun cans, they are also suitable for in-processthickness measurement of ferrous cans. The second embodiment in FIGS.10, 11 is suitable to measure the thickness of ferrous or non-ferroustubing, since the individual polar sensors PS and driving cores 55 donot induce circulating currents in the tubing because there is no closedmagnetic loop around the tube specimen. As aforementioned, the polarsensor in an inclined axis mode may also be used to measure the hardnessof ferrous metals. The remanence effect of the hardness causes a laggingphase angle as shown in FIG. 7D. Although the inclined axis polar sensorhas been shown measuring the sidewall thickness of cylindrical articlessuch as containers and tubing, flat sheet material such as steel mayalso be measured for thickness and hardness. The testing procedure isthe same as shown in FIG. 3, where the Z-axis of the polar sensor PS isinclined at an acute angle to the workpiece metal sheet 83 (ferrous ornon-ferrous). Several inclined axis polar sensors may be arranged in arow traverse to a moving sheet of metal such as aluminum foil to measurethickness during in-process manufacture (not shown). FIG. 15 is anexample signal processing circuit for all the embodiments of thisdisclosure. The output signal from a polar sensor is fed to thedifferential amplifier 113, from there to a precision zero crossingdetector 114. From zero crossing detector 114 the extracted phase angleis sent to a microprocesor unit 115 for phase comparison to sine-cosineexcitation reference signal 118. Microprocessor 115 generates a feedbackcontrol signal 116 which may control the ram and die centering. Sidewallthickness and concentricity information may be generated on output 117,for display. The poly-phase polar sensor signals are connected todifferential amplifiers 113, 119, and phase comparison is made inmicroprocessor 115. Microprocessor 115 may also contain an amplitudepeak detector for measuring sensor to workpiece spacing. Any minor phaseerrors resulting from sensor to workpiece spacing variation may becompensated for by suitable circuitry included in the signal processingmeans by utilizing the amplitude component of the polar coordinatesignal. This microprocessor generated compensation signal should reduceany "liftoff" error to a very small factor.

I claim:
 1. Apparatus for measuring the sidewall thickness of anon-ferrous cylindrical article having a central longitudinal axis withx-y centering coordinates during an in-process ram and die drawingoperation, the said apparatus comprising:a) a multiple polar sensorassembly for inducing eddy currents in the said article sidewallgenerating polar coordinates signals by the angular motion of a rotatingmagnetic field, each polar sensor coupling a rotating magnetic field tothe workpiece and generating a sine wave signal, the phase angle ofwhich primarily represents article sidewall thickness and the amplitudeprimarily represents sensor to workpiece distance, first and secondpolar sensors being disposed on the x-axis of the cylindrical article,third and fourth polar sensors disposed on the y-axis of the cylindricalarticle, said multiple polar sensor assembly further comprising:i) ahollow toroid core formed of a high permeability ferromagnetic materialhaving four bores in the inner circumference wall on the said x-y axes,the hollow toroid core also having an elliptical shaped toroidalcross-section, the minor elliptical axis of the toroidal cross-sectionbeing disposed at an acute angle to a plane drawn perpendicular to thecentral axis of the hollow toroid core, the said acute angle forming thesides of a cone, the said plane forming the base of the cone, the axesof said bores being disposed on the sides of the cone and on radiallines radiating from the central axis of the hollow toroid core, andalso being disposed on the said minor elliptical axis; ii) the fourpolar sensors being mounted partially within the four bores as toprovide an extending portion outside the surface of the hollow toroidcore for coupling the rotating magnetic field to the workpiece; iii) afirst excitation winding wound within the hollow toroid core forinducing a first magnetic field throughout the hollow toroid core; iv) asecond excitation winding wound around the outside of the hollow toroidcore, being subdivided and wound between the extending portion of themounted polar sensors for flux symmetry, said second excitation windingfor inducing a second magnetic field throughout the hollow toroid core;v) sine-cosine excitation being applied to the first and secondexcitation windings for inducing a rotating magnetic field throughoutthe hollow toroid core; vi) the hollow toroid core being separable forassembly; vii) signal processing means to receive the polar coordinatessignals to determine the phase angle and amplitude indicating thethickness of said cylindrical article sidewall and determining the ramand die centering on the x-y axes of the cylindrical article.
 2. Theinvention according to claim 1, wherein each of the polar sensorscomprise:a) a pick-up core formed of a high permeability ferromagneticmaterial, further comprising:i) a central cylindrical magnetic pole, acylindrical outer magnetic pole concentrically surrounding the centralcylindrical magnetic pole and spaced apart to provide a pick-up coilspace and: a base portion for connecting these magnetic poles at oneend, the end opposite the base portion forming an annular sensing face,said annular sensing face being perpendicular to the axis of the centraland outer magnetic poles, the sensing face for coupling the rotatingmagnetic field to the sidewall of the cylindrical article; ii) a pick-upcoil wound around the central magnetic pole, and being disposed coplanarto the rotating magnetic field for a signal null when no target ispresent.
 3. The invention according to claim 1, wherein the axis of themounted polar sensors are inclined at an acute angle to a plane drawnperpendicular to the central axis of the hollow toroid core to providean unbalanced flux coupling to the cylindrical article sidewall andgenerating a signal in the pick-up coil having a phase anglerepresenting the sidewall thickness and an amplitude in porportion tosensor-workpiece spacing.
 4. The polar sensor according to claim 2,wherein the said polar sensor further comprises:i) a conical sensingface wherein the central cylindrical magnetic pole is elongated forminga cone concentric with the outer cylindrical magnetic pole for greaterflux coupling to the workpiece.
 5. The invention of claim 1, furtherdefined as a ferrous cylindrical article sidewall thickness measurementapparatus, the resultant signal phase angle differing by approximately180 degrees compared to the non-ferrous workpiece signal.
 6. Theinvention according to claim 2, wherein the said polar sensor generatesat least two polar coordinates signals sinultaneously for greaterthickness measurement resolution, said sensor further comprising:a) apick-up core formed of a high permeability ferromagnetic material,further comprising:i) a central first pole around which is wound a firstpick-up coil for generating a first polar coordinates signal; ii) acylindrical second pole concentrically disposed around the first poleand first pick-up coil; iii) a second pick-up coil being wound aroundthe cylindrical second pole for generating a second polar coordinatessignal; iv) an outer cylindrical third pole disposed concentricallyaround the cylindrical second pick-up pole and second pick-up coil; v) abase portion for connecting these three poles.
 7. The invention asdefined in claim 6, further comprising:i) a conical sensing face whereinthe central first pole and the cylindrical second pole are elongatedforming a cone concentric with the outer cylindrical pole for greaterflux coupling to the workpiece.
 8. A method of utilizing four polarcoordinates sensor assemblies for measuring the sidewall thickness of anon-ferrous cylindrical article having a central longitudinal axis, thearticle also having x-y axes for centering, during an in-process ram anddie drawing operation, the said method comprising the steps of:a)arranging four polar sensor assemblies on the said x-y axes in proximityto the article sidewall outer surface there being: b) a first and secondpolar sensor assembly disposed on the x-axis and: c) a third and fourthpolar sensor assembly disposed on the y-axis; each of the said polarsensor assemblies comprising: d) a hollow toroid driving core formed ofa high pereability ferromagnetic material, and having a bore in theouter circumference wall, the axis of said bore being perpendicular tothe central axis of the hollow toroid driving core; e) a polar sensorpick-up assembly mounted partially within the bore, the extendingportion outside the driving core wall forming f) a sensing face forcoupling the rotating magnetic field to the specimen article sidewallfor eddy current induction; g) a first excitation winding wound withinthe hollow toroid core for inducing a first magnetic field throughoutthe hollow toroid core; h) a second excitation winding wound around theoutside of the hollow toroid core, being wound symmetrically to themounted polar sensor pick-up for flux balance, the second excitationwinding for inducing a second magnetic field throughout the hollowtoroid core; i) sine-cosine excitation being applied to the first andsecond excitation windings for inducing a rotating magnetic fieldthroughout the hollow toroid core, said field having distributive axesperpendicular to the surface of the hollow toroid core at all points; j)each said polar sensor pick-up assembly further comprises: k) a pick-upcore formed of a high permeability ferromagnetic material, assemblyfurther comprising: l) a central cylindrical magnetic pole, acylindrical outer magnetic pole concentrically disposed around thecentral cylindrical magnetic pole, and spaced apart to provide a pick-upcoil space, and a base portion for connecting these magnetic poles atone end, the end opposite the base portion forming an annular sensingface, said annular sensing face being perpendicular to the axis of thecentral and outer magnetic poles; m) a pick-up coil wound around thecentral magnetic pole, said pick-up coil being coplanar to the rotatingmagnetic field and flux balanced for a signal null when no workpiece ispresent; n) the annular sensing face axis being disposed at an angle tothe cylindrical article sidewall outer surface for an unbalanced fluxcoupling to the pick-up coil generating a polar coordinates signal, thesignal phase angle compared to the sine-cosine excitation represents thesidewall thickness and; the signal amplitude primarily representingsensor to workpiece spacing; o) signal processing means to receive thegenerated polar coordinates signals for determining sidewall thickness.9. The method according to claim 8, wherein the annular sensing faceaxis is disposed at an acute angle to a plane drawn perpendicular to thelongitudinal axis of the cylindrical article for unbalanced fluxcoupling to the pick-up coil.
 10. The method according to claim 8,further defined as a ferrous cylindrical article sidewall thicknessmeasurement method, wherein the generated signal phase angle isapproximately 180 degrees out of phase compared to the non-ferrousarticle signal.
 11. The method as define in claim 8, wherein the saidpick-up assembly is further defined as a poly-phase pick-up assembly forgenerating at least two polar coordinate signals simultaneously, saidassembly comprising:a) a pick-up core formed of a high permeabilityferromagnetic material, further comprising:ii) a central first polearound which a first pick-up coil for generating a first polarcoordinates signal; iii) a cylindrical second pole concentricallydisposed around the first pole and the first pick-up coil; iv) a secondpick-up coil being wound around the cylindrical second pole forgenerating a second polar coordinates signal; an outer cylindrical thirdpole disposed concentrically around the cylindrical second pole andsecond pick-up coil; a base portion for connecting these three poles.12. The method as defined in claim 8, being further defined as a ferrouscylindrical article thickness measurement method, wherein the generatedpolar coordinates signals differ by approximately 180 degrees comparedto the non-ferrous signal.
 13. The method as defined in claim 8, beingfurther defined as a non-ferrous sheet article thickness measurementmethod, further comprising the steps of:i) arrangingat least one of thesaid polar coordinates sensor assemblies in proximity to the non-ferroussheet article, the axis of the annular sensing face being disposed at anacute angle to a line drawn perpendicular to the plane of the said sheetarticle for unbalanced flux coupling to the pick-up coil; ii) processingthe generated polar coordinates sigal to determine specimen thickness bymeans of the signal phase angle.
 14. The method as defined in claim 13,being further defined as a ferrous sheet article thickness measurementmethod, wherein the generated polar signal differs in phase byapproximately 180 degrees as compared to the non-ferrous specimensignal.